I think Definitive Technologies is a very good brand too. If you're still not happy with the power output, check with the store you bought them at about an upgrade policy. The places I have looked at have a one year trade up where you can return the speakers you bought and put the money you paid for them into a more expensive set. This was huge for me when I decided I could do better than Bose 701. I traded those in for $900 (I bought them on sale) for Mirage OM-5 floor models marked down to $2000. So I paid $1100 for $3500 speakers! The two places I've shopped are Tweeter and Sound Forum.

I know with Definitive Tech towers and surrounds, all you have to do is match the midrange driver, ie all speakers should have 4", 5.25", or a 6.5" midrange. This applies to power towers, unpowered, surrounds and the center channels. So in your case, you should be fine with anything that has the 5.25" midrange.

I think I would agree with JasonA that the amp is a bit strong for these speakers. DefTechs go 4-8 ohms impedance and, to my understanding, the amp will run at 4 ohms giving 300 Watts to each speaker. If I'm wrong the amp stays at 8 ohms, I think it would be a fantastic match.